1. Field of the Invention
The present invention generally relates to positron emission tomography (PET) and magnetic resonance imaging (MRI) technologies. More specifically, the present invention relates to a system and a method that integrates PET and MRI technologies into a combined scanner capable of simultaneous PET and MRI imaging.
2. Related Art
Positron emission tomography (PET) is a powerful molecular imaging modality that uses positron-emitting radionuclides attached to biologically relevant molecules to provide exceptionally sensitive assays of a wide range of biological processes. PET imaging is commonly used to diagnose cancer and to examine the effects of cancer therapy by characterizing biochemical changes in the cancer tissues. PET imaging has also been effective in detecting coronary artery diseases, brain disorders and other diseases.
Unfortunately, PET imaging has a serious drawback: the images produced by a PET scanner typically have relatively poor spatial resolution. This poor spatial resolution is the result of fundamental physical limitations of the PET process, which involve the mean-free-path of the positrons emitted by the radionuclides (i.e., positron range) and the non-colinearity of the two coincidence photons produced from a positron-annihilation event. Consequently, for many types of radiotracers used in the PET imaging process, their associated PET images often provide limited anatomical information, making unambiguous localization of the sources of the positron-emissions extremely difficult.
Note that the lack of spatial resolution in the PET images can affect the accuracy in quantifying PET data, which can cause significant underestimation of the actual isotope concentration in structures smaller than 2× the spatial resolution of the PET scanner. This inaccurate quantification of PET data can subsequently cause problems, for example, in interpreting PET images of tumors where a decrease in uptake of a radiotracer following treatment could indicate tumor shrinkage, a change in the biological function measured by the radiotracer, or both.
On the other hand, magnetic resonance imaging (MRI) is a widely utilized imaging technique that provides exquisite high-resolution anatomical information in the sub-millimeter range. Furthermore, MRI facilitates access to a range of physiologic parameters (e.g. water diffusion, permeability, vascular volume), and through spectroscopic imaging, to spatially-localized metabolic and biochemical information.
Because of these unique properties of the two imaging techniques, PET and MRI are largely complementary in the information they provide and merging these two modalities in the study of experimental animal models will allow us to exploit, in a synergistic fashion, the strengths of both techniques. Moreover, the accurate registration of simultaneously acquired MRI and PET images not only facilitates the anatomic localization of PET signals, but also provides information that can lead to improved quantification of the PET images through accurate attenuation correction (based on segmentation of the MR into different tissues types and assigning known tissue attenuation values), model-based estimates of scatter, and, most importantly, the potential for partial volume correction.
However, the task of integrating the two imaging modalities for simultaneous PET and MRI imaging presents many challenges. In particular, the PET scanner, which is typically the smaller modality of the two, will most likely be placed inside the MRI. In other words, the PET scanner will be immersed in the typically high magnetic field environment of the MRI. This can cause problems when combining PET and MRI, because there is a high probability of interference or interaction between the two systems in the form of electromagnetic interference (EMI). More specifically, the integrated PET-MRI system requires the PET detectors to work and work well in the high magnetic field environment. However, most of the photon detectors and associated electronics contain metal components and their performance is consequently sensitive to magnetic fields and electromagnetic signals. On the other hand, these metal components, when immersed in the magnetic field, can become magnetized and can subsequently introduce a magnetic field inside the MRI scanner, which can disturb the homogeneity of the main magnetic field and the associated gradient fields within the MRI scanner. Hence, these EMI effects, between the PET components and the MRI components, can cause potentially serious artifacts and reduce signal-to-noise ratio (SNR) in both the PET and MRI images.
Another challenge involved in combining PET and MRI imaging mechanisms relates to constructing a compact PET scanner that fits within a limited space inside the bore of the main magnet of the MRI scanner with high precision, so that the PET scanner is precisely aligned with the detectors of the MRI scanner, which allows the simultaneously generated PET and MRI images to have accurate registration.
Currently, there are a few approaches which are being investigated for combined PET-MRI systems. One of these approaches is to use 3-5 meter long optical fibers to couple scintillator elements placed inside the magnet of MRI to photomultiplier tubes (PMTs) and associated electronics located outside of the magnetic field. This approach is illustrated in FIG. 1A. Note that the long optical fibers are required because of the high sensitivity of PMTs to even small magnetic fields. By placing only the scintillator material inside the MR scanner, and keeping all of the PET readout electronics outside of the magnet, any EMI between the two imaging systems can be minimized.
However, there are several drawbacks to this approach. First, by using 3-5 meter long optical fiber to transmit the optical signal, a significant fraction (somewhere between 50% and 75%) of the scintillation light is lost, causing a deterioration in crystal identification, energy resolution and timing resolution in comparison to photon detectors that are directly coupled to the scintillators. A second problem is that achieving both high spatial resolution and high sensitivity in the PET image requires a large number of detector channels. However, because of the limited space inside conventional MR magnets, it is not practical to fiber-optically couple large numbers of fibers to external electronics.
A similar fiber-PMT-based approach for combining PET and MRI uses a split magnet low-field MR system. The split-magnetic approach allows a relatively large number of PET detectors to be placed inside the gap within the split magnets of the MR system, while also reducing the fiber lengths significantly compared with the single-magnet systems. Unfortunately, this approach has the drawback of requiring a specialized, lower-field magnet, which significantly limits the applicability of the combined system.
Another approach uses magnetic field-insensitive solid-state photon detectors—avalanche photodiodes (APDs), as replacements for PMTs, and couples these APDs directly to the back of the scintillator elements (FIG. 1B). Note that APDs are relatively immune to magnetic fields and have been demonstrated to work inside MRI scanners at fields as high as 9.4 T. A typical APD-based setup also requires a charge-sensitive preamplifier (CSP) to be placed as close as possible to the detector to minimize the capacitance, thereby ensuring lower noise and better signal quality. In addition, to shield the PET electronics from external high frequency signals from the MRI, these electronics have to be enclosed in metal housing. This approach solves the many limitations of fiber-optically coupled systems. However, placing many metal components within the central region of the MRI system introduces inevitable EMI between the main magnetic field, the RF coils and gradient coils of the MRI system, and the PET electronics. It is also questionable whether artifact-free simultaneous PET and MRI images can be acquired with such an approach.
Hence, what is needed is an integrated PET-MRI system that minimizes electromagnetic interference between the PET components and the MRI components, while taking advantage of the magnetic field insensitivity of photodetectors to produce high-resolution, high-sensitivity PET images without the above-described problems.